This invention relates to a double-tube vibration damper with a bottom valve unit between the lower working chamber remote from the piston rod. The bottom valve unit controls the flow of liquid between the lower working chamber and the reservoir chamber both in normal operation and when the piston rod approaches its innermost position. When the piston rod approaches its innermost position the bottom valve unit overtakes the function of a hydraulic abutment preventing mechanical impact of the piston rod on a mechanical abutment of the cylinder. The design of such a bottom valve unit is therefore a highly complicated problem both with respect to the function, long-term use and mass production.